An automatic transmission for an automobile is typically equipped with a two to five speeds planetary gear transmission mechanism in addition to a torque converter in the form of a fluid coupling, and gear change is performed by appropriately fixing and releasing a sun gear and planetary gears etc. in the planetary gear transmission mechanism by frictional engagement means such as a clutch or a brake. In some cases, the frictional engagement means internally built in the automatic transmission is internally equipped with a one way clutch to allow free rotation of the gear shaft etc. in one rotational direction, to thereby facilitate gear shift control.
This type of one way clutch includes an inner ring, an outer ring disposed coaxially with the inner ring, a plurality of cam surfaces provided along the circumferential direction on the inner circumferential surface of the outer ring or the outer circumferential surface of the inner ring, each including a recessed portion and a slope, torque transmission members provided between the outer circumferential surface of the inner ring and the aforementioned cam surfaces, biasing members that bias the respective torque transmission members against the aforementioned slopes of the cam surfaces, and a retainer attached on the outer ring to retain the torque transmission members and the biasing members, as described, for example, in Japanese Patent Application Laid-Open No. 2001-59530.
In this structure, the biasing member biases the torque transmission member against the slope of the cam surface. While in one rotational direction the torque transmission member acts integrally with the inner ring and the outer ring to transmit torque, in the reverse rotational direction the torque transmission member compresses the biasing member and is received in the recessed portion of the cam surface to allow relative rotation of the inner ring and the outer ring, whereby the driving ring, whether the inner ring or the outer ring, rotates freely so that no torque is transmitted.
Upon transition from free or idle rotating of the inner ring to torque transmission, in other words, upon transition from the disengaged state to the engaged state, it is necessary for the oil film between the torque transmission member and the contact potions of the inner ring and the outer ring to be broken so that direct contact of the metal parts is achieved.
However, in the case of the above described prior art structure, if the one way clutch is used in a very low temperature (e.g. −20° C. to −40° C.) environment in which lubricant oil or automatic transmission fluid (ATF) used in the automatic transmission has a high viscosity, the torque transmission member cannot break through the oil film on the contact portions of the inner and outer rings at the time when engagement with the inner and outer rings is to be achieved. Thus, there may be cases where normal engagement is not achieved, and the torque transmission member continuously slides on the oil (ATF) having a high viscosity.
To avoid such sliding in a very low temperature environment, in the roller type one way clutch disclosed in Japanese Patent Application Laid-Open No. 2006-2925, the sliding surface of the roller is designed to have a concave portion that is recessed with respect to the axial direction along its entire circumference, whereby breaking of the oil film at the moment of transition from free or idle rotation to the start of engagement in the early stage of engagement is facilitated, and metal parts can easily come in contact with each other. By causing the initial contact of the metal parts in this way, the engagement performance in a low temperature environment is enhanced.
As another countermeasure to the sliding in a very low temperature environment, a stronger biasing force may be exerted on the torque transmission member to increase the pressure on the contact surface between the torque transmission member and the cam surface or the contact surface between the torque transmission member and the track surface, to thereby facilitate breaking of the oil film. In this case, however, a dragging torque during idle rotation is also increased, which lead to other problems, such as promotion of abrasion of the torque transmission member and the inner or outer ring and a decrease in the efficiency of the automatic transmission.